With piezo-electric materials when an electrical field is applied across them a mechanical strain results. This mechanical strain results in expansion or contraction of the piezo-electric material. Accordingly piezo-electric materials can be used to provide mechanical movement in response to an applied electric field. They are particularly used where only small movements are required, for example, on the stages of scanning electron microscopes or in micro-manipulators.
According to this invention an opto-mechanical device comprises an optically responsive actuating member in which a mechanical strain occurs in response to changes in its illumination.
The optically responsive actuating member is preferably at least partly formed from a semiconducting glassy matrix. Preferably the semiconducting glassy matrix includes a group VI element alone or in combination with at least one element from the same or a different group of the periodic table. Such material is often referred to as a chalcogenide glass. Whilst the elements forming the semiconducting glassy matrix may be in stoichiometric proportions it is preferred that they are in non-stoichiometric proportions. Other materials such as photo-active polymers which demonstrate photo-anisotropy can also be used.
Preferably the opto-mechanical device includes means to illuminate the actuating member. The illumination means may be formed by an optical waveguide or may include a light emitting source arranged to illuminate the actuating member directly or via a coupling fibre or lens, or may simply be a window in an otherwise opaque housing or package containing the device to allow light to impinge upon the actuating member. When the illumination means includes an optical waveguide the illuminating light may be evanescently coupled to the actuating member or the output from the waveguide used to illuminate the actuating member. It is also possible for the actuating member itself to act as a waveguide and, in this case, the illumination means may be coupled to the actuating member.
The mechanical strain in the actuating member resulting from its illumination is provided by two different effects. Firstly there is a scalar effect in which when an initially isotropic semiconducting glassy matrix is illuminated by unpolarised light it expands. This expansion produces a metastable state and the semiconducting glassy matrix can be returned to its isotropic state by heating. An opto-mechanical device using this scalar effect has only limited application.
The inventors of the present application have discovered that there is also an additional vectorial change in the mechanical strain generated in the actuating member when it is illuminated with polarised light. When the semiconducting glassy matrix is illuminated with polarised light having its electric vector in a predetermined direction this produces a strain in the matrix which causes it to contract in the predetermined direction. Conversely, illuminating the semiconducting glassy matrix with light polarised to have its electric vector orthogonal to the predetermined direction produces a strain in it which causes it to expand in the predetermined direction. This expansion or contraction is again a metastable state of the actuating member and this metastable state can once again be relieved by the application of heat. However, much more usefully the metastable state can be changed by merely changing the orientation of the polarisation of light being used to illuminate the actuating member. Accordingly, by changing the state or angle of polarisation of the illuminating light a corresponding change in the mechanical strain of the actuating member occurs and hence a corresponding change in its contraction or expansion can be made to occur. This change is completely reversible and repeatable and there is, for example, no hysteresis as occurs in piezo-electric actuating members.
Preferably therefore the opto-mechanical device also includes means to illuminate the actuating member with a source of polarised light and preferably includes means to vary the angle of polarisation of the light applied to the actuating member. In this case the means to illuminate the actuating member is preferably polarisation-preserving and may include a polarisation-preserving optical fibre and may include an electro-optical modulator to vary the angle of polarisation of the light applied to the actuating member. Other ways of changing the angle of polarisation of the light applied to the actuating member include switching between light of perpendicular polarisation states by switching between differently polarised light sources or by, for example, switching a half wavelength plate into the optical path.
The linear expansion or contraction of the semiconducting glass matrix on its illumination is only small, for example of the order of 1:200,000. The change also only occurs in regions which are illuminated by the light and so tends to occur only in a reasonably shallow surface layer of the glassy matrix into which the illuminating light penetrates. It is believed that, in the case of chalcogenide glasses, the effect is caused by the incoming photons of the illuminating light exciting one or more of the lone-pair electrons in the p-orbital of the outer shell of the group VI element and, having excited these electrons, an electron-hole pair is created which no longer has the spatial symmetry of the lone-pair orbital and, due to the change of interatomic potential leads to a swing of the chalcogen atom resulting in a local distortion of the glass leading to expansion or contraction.
Inevitably the light is absorbed by the glass and its energy is transferred to the excitation of the lone-pair electrons. Accordingly the light is not able to penetrate far into the glassy matrix before being absorbed. If reasonably strongly absorbed it typically penetrates to a depth of about 1 xcexcm.
It is possible for the actuating member to be substantially entirely formed by the semiconducting glassy matrix and in this case a simple change in length of the actuating member occurs as a result of changes in its illumination. Since the change in length is only small this change in length is difficult to detect and certainly difficult to use directly. Preferably therefore, the actuating member includes a substrate which is unaffected by changes in illumination. Preferably the actuating member is formed by a layer of the semiconducting glassy matrix bonded to the substrate. Preferably the thickness of the semiconducting glassy matrix and the substrate are comparable with one another. In this case when the layer of semiconducting glassy matrix expands or contracts upon changes in its illumination, its expansion or contraction relative to the substrate causes the actuating member to bend in much the same way as a bi-metallic strip bends in response to changes in temperature. This results in a very much greater mechanical displacement of the actuating member than that resulting from the expansion or contraction of the semiconducting glassy matrix alone. For example by mounting such an actuating member as a cantilever, the free end of the cantilever is capable of moving a distance a few orders of magnitude greater than the simple change in length of the semiconducting glassy matrix. Equally if the actuating member is mounted so that it is constrained at both ends it bows and flattens with changes in illumination and its central portion again moves a distance which is considerably greater than that of the change in length of the semiconducting glassy matrix.
The opto-mechanical device in accordance with this invention may be used as a sensor to detect the presence of light or its polarisation state with the resulting mechanical strain in the actuating member being used to indicate the presence or polarisation state of the light. The mechanical strain of the actuating member may be monitored directly or preferably, monitored via a secondary system. Another use of the opto-mechanical device in accordance with this invention is to create a mechanical movement to do direct work, for example by using the actuating member as part of a micro- or nano-manipulator and using the change in size of the actuating member to move a workpiece being handled by the micro- or nano-manipulator. A further application is to use the opto-mechanical device to perform a control function on a secondary system and thereby control the secondary system in response to changes in the light illuminating the actuating member.
The secondary system may be any one which responds to mechanical strain or movement and thus, for example, the secondary system may include an optical lever comprising a laser beam which is reflected from the actuating member or an element connected to it and then impinges upon a position-sensitive photo-detector so that as the actuating member moves or bends the effect of its movement turns the optical lever formed by the laser beam so that it moves along the position-sensitive photo-detector. The output of the position-sensitive photo-detector then gives an indication of the mechanical movement of the actuating member and hence of the illumination applied to it. The actuating member may include a metal electrode and in this case the mechanical movement of the actuating member and its associated electrode towards or away from an associated electrode provides a change in capacitance between the two electrodes. This change in capacitance may be monitored to give a representation of the mechanical movement of the actuating member and hence of the illumination applied to it. Equally, the substrate may be formed by a piezo-resistive material the resistance of which changes as a result of a strain imposed upon it. Accordingly, in this case, as a result of a change in strain in the substrate caused by the strain in the semiconducting glassy matrix, the electrical resistance varies so resulting in a change in resistance being used to provide an indication of the change in strain of the semiconducting glassy matrix and hence its illumination. A further way in which the actuating member can be used to control a secondary system is in the case where the secondary system is a fluidic system. In this case the actuating member may be arranged close to a nozzle in a fluidic system so that as the illumination of the actuating member varies and hence its displacement varies, it gets closer to or further away from the nozzle so varying the rate of flow through the nozzle and hence the back pressure upstream of the nozzle. In this way the opto-mechanical device can be used to influence a fluidic system and thus produce a light sensor or a controller which is responsive to the light input into the device which, being completely free from electrical contacts, would be intrinsically safe in an explosive environment.
Not only can the actuator do direct work as a micro-manipulator it is also possible for it to do direct work as the actuating member of a pump. An example of this is where the semiconducting glassy matrix is formed on a planar substrate so that with changes in its illumination the actuating member bows. By changing the illumination of the actuating member so that it alternately bows and flattens it can be used as a diaphragm of a diaphragm pump. It is also conceivable that it can be used as an optically powered motor by taking the reciprocating movement either from such a diaphragm or from a cantilever arm and converting this into a rotation.
When the elements forming the semiconducting glassy matrix are in non-stoichiometric proportions the exact composition of it can be tailored to influence and suit its intended use. Firstly, by selecting and varying the proportions of the element or other elements in the semiconducting glassy matrix the band gap of the semiconductor can be varied and so matched to the energy of the light which is used to illuminate the actuating member. Clearly this can be used to make the actuating member selective to light of a predetermined wavelength where the actuating member is used to detect light of a predetermined wavelength or alternatively merely to match the actuating member to the wavelength of light output by a light source used with the opto-mechanical device and thereby optimise its sensitivity. It is also possible to change the rigidity of the glass material and, in this way affect the amount of expansion and contraction that occurs. For example, introducing iodine into the semiconducting glassy matrix appears to make it less rigid and so causes greater expansion and contraction of the matrix for a given change in illumination. Equally, the introduction of germanium into the semiconducting glassy matrix appears to make it more rigid and so results in a lower expansion and contraction for the given change in illumination.
Another way in which the composition of the semiconducting glassy matrix can be tailored to be given particular advantages is by changing its composition so that, for example, its thermal expansion is substantially the same as the thermal expansion of any substrate used in the actuating member. In this way the actuating member can be made insensitive to thermal changes so it only responds to changes in illumination. Equally, by changing the composition, the scalar effect of illumination can be enhanced or suppressed. Clearly where it is the response of the opto-mechanical device to the scalar effect of illumination which is used it is required to increase the sensitivity of the device to the scalar effect and, reduce as far as possible, the effect of any change in polarisation whereas, more usually, where it is the response to a particular polarisation state that it is required it is important to suppress, as far as possible, the scalar effect.
Typically the semiconducting glassy matrix may be formed by melting the ingredients in bulk and then subsequently working the melt by conventional glass working techniques to provide the actuating member. Particularly where it is to be formed as a thin film on a substrate it may be formed by sputtering, chemical vapour deposition or by evaporation. One chalcogenide glass which has shown good results when used as the semiconducting glassy matrix is As50 Se50 particularly when this is combined with a substrate of silicon nitride.